Transition-metal-mediated XÀY sbond-breaking and bond-formation reactions (X and Y = group 3-6 elements, Scheme 1) are among the current highlights of organometallic chemistry. [1] It is generally assumed that these reactions occur through formation of the intermediate XÀY s complexes 1, and therefore the study of such species might allow an insight into the bond formation/rupture processes within the coordination sphere of a transition metal. [2] Much of the work has been carried out with complexes that have at least one hydrogen atom (X = H), which provokes questions regarding whether the isolation of "hydrogen-free" X À Y s complexes is achievable. [2b] The unique propensity of hydrogen to participate in the complexation of s bonds is attributed to the spherical symmetry of its 1s orbital, whereas for other elements complexation of an XÀY s bond composed of highly directed sp x -hybridized orbitals would require a substantial reorganization energy. [1b] This simple argument suggests that the best candidates for s-bond complexation are heavy elements of groups 3-6, as a result of the more diffuse nature and steric accessibility of their s bonds.Surprisingly, the first evidence for "hydrogen-free" s-bond complexation was observed with the small and relatively rigid carbon atom. It has been reported recently that the long-known b-agostic CÀH bonding motif is not derived from CÀH!M electron donation, but rather from a M···C b interaction with a metal. [3] In addition, the weak complexation of a CÀC bond to a metal atom has been proposed for a Ti complex. [4] Very recently, agostic CÀC interactions have been discovered in a rhodium complex, [5] where the effect is promoted by steric strain, and there have also been reports regarding the occurrence of agostic CÀSi bonds. [6] Although providing some hope, these earlier studies were inconclusive as to whether a genuinely hydrogen-free s complex was possible. It was perhaps not surprising that a breakthrough came from the field of transition-metal silicon chemistry, as the SiÀSi bond is much weaker and more accessible than the CÀC bond.In 2001, the research group of Tanaka reported that the 1,2-disilylbenzene 2 reacts with [Ni(dmpe) 2 ] (dmpe = 1,2bis(dimethylphosphanyl)ethane) to give the dimeric bis(silyl)nickel(ii) complex 3, which upon heating at 110 8C rear-X Y X Y X Y L n M L n M + 1 L n M Scheme 1. Reversible behavior of transition metal XÀY s complexes.